1. Field of the Invention
The present invention relates to a current-regulated driver circuit for light sources, in particular LED light sources.
2. Description of the Related Art
Such circuits typically include LED light sources and an electronic control unit (ECU) suitable for regulating a driver current absorbed by the LED light sources, which may be arranged in LED strings or matrixes. More specifically, the electronic control unit includes a reference circuit of an electric quantity and a regulation circuit of the driver current. The reference circuit of an electric quantity provides a reference of an electric quantity, such as a reference voltage Vref; the current regulation circuit imposes a specific driver current on the light sources, on the basis of the reference of the electric quantity provided by the reference circuit of electric quantity and on the value of an electric resistor known in the art as a bin resistor. In some applications, for example in LED lighting for vehicle lights, the electronic control unit and LED light sources are generally placed on separate electronic circuit boards.
Such LED light sources however are supplied by the manufacturers and are grouped in lots according to different luminous flow selections (or binning. The LEDs from each, when driven at nominal voltage and/or current values, emit a variable luminous flow only within a specific and limited predefined range. As a result, a light of a first vehicle light, such as the right light, may be made with a lot of LEDs having a first flow selection mounted on a first LED circuit board, while a second vehicle light, such as the left light, may be made with a second lot of LEDs having a second flow selection. Obviously, such same light, whether of the first or second vehicle light, such as for example a brake light, side light, fog light, reverse light, indicator light, dipped beam headlight, full beam headlight or the like, must emit the same luminous flow regardless of the LED lot used. The same consideration applies to vehicle lights installed on different, similar models of vehicle. In practice, the light manufacturer chooses the lot with the lowest flow selection for a light and limits the luminous flows of the LEDs of the other lights to emitting the same luminous flow, reducing the power supply current on the basis of information generally provided by the bin resistor value.
In one embodiment frequently used in the related art, the driver circuit of light sources has the configuration represented schematically in FIG. 1, which shows the bin resistor (RBIN) mounted on the LED circuit board and connected to the electronic control unit (ECU) mounted on another circuit board.
One drawback of this circuit is the need to position and connect two cables (W1, W2) to detect the current on the bin resistor. Moreover, since the bin resistor is on the LED circuit board and the electronic control unit is on another circuit board, the connection cables and connectors introduced may give rise to problems of electromagnetic compatibility. For the same reason, the feedback loop of the current regulation module of the ECU may become unstable on account of the onset of capacitive and inductive components introduced by the connection of the two cables W1 and W2. In fact, the voltage drop on the bin resistor is a modest value, so that even the smallest disturbance may significantly influence the total current flowing in the LEDs. Moreover, given that the bin resistor value Rbin is relatively small, relatively small impedance values introduced by the connections of the cables W1, W2 may significantly influence the total current flowing in the LEDs.
The transmission line between the LED terminal strip and electronic control unit can cause a variation in the current flowing in the LEDs. If the bin resistor must stay on the LED terminal strip and is connected to ground and to the feedback circuit by a transmission circuit, such transmission circuit introduces parasitic resistive, inductive, and capacitive elements. The resistance component is created by the connectors of the two electronic circuit boards and by the resistance of the connector cables between the circuit boards. Moreover, oxidation of the connectors also causes a variation in their resistance. The capacitive and inductive components are related to the length of the cables, which may pick up disturbances coming from the outside environment. Such electromagnetic disturbances may be identified as a voltage variation ΔVEMC. Such voltage variation, to the order of mill volts, thus depends solely on external conditions and is introduced on the bin resistor line.
Consequently, while on the emitter of the driver transistor there is a fixed reference voltage Vref, on the bin resistor there is the reference voltage Vref plus the disturbance ΔVEMC. So, the bin resistor current, IRBIN, and therefore the current flowing in the LEDs, ILED, is given by (Vref+ΔVEMC)/RBIN. Considering also the contribution of the resistance of the connectors RT, one has:ILED=(Vref+ΔVEMC)/(RBIN+RT).
So, ILED no longer depends solely on Vref and on RBIN, but on Vref, ΔVEMC and RT. With a Vref for example of 0.5 V, even small disturbances significantly influence the ILED. Even the bin resistor, typically to the order of 1-10 ohm, is influenced by the connector resistance, for example due to the oxidation of the connectors.
In addition, as the above, the reactive components LC introduced in the feedback loop may cause instability and the oscillation of the feedback circuit.
Published EP patent application No. EP1411750A2 describes a power supply circuit of an LED lighting unit which uses an identification resistor having a resistance corresponding to the characteristics of the LED circuit. In one embodiment, the power supply circuit includes an identification portion which measures the resistance of the identification resistor included in the LED circuit, determines which range the resistance measured belongs to, and provides in output a classification signal based on such determination. A circuit control portion of the constant current receives the classification signal, establishes a maximum admissible current depending on such classification signal and provides a driver current to the LED circuit proportional to a predefined current value within the maximum admissible value.
In the embodiment, the identification resistor has a terminal connected to a constant voltage power supply generator. The range which the resistance of the identification resistor belongs to is determined by comparing, by a plurality of comparators, the voltage on the other terminal of the resistor with a plurality of constant voltage references.
Such circuit performing comparison of the voltage values is not however immune from electromagnetic disturbances and requires a constant power supply generator to connect the identification resistor to. For example, an electromagnetic disturbance which is propagated along the cable connecting the identification resistor and the voltage comparison circuit could easily cause an alteration of the voltages to be compared and thus cause an error in the determination of the range of resistance values.
Consequently, the circuit described in EP1411750A2 is not suitable for applying in situations, such as in the case of a vehicle light, where the power supply voltage is highly variable and where significant electromagnetic disturbances are present. It is to be noted, for example, that the driver circuit of a vehicle light is powered by a battery and by an alternator which provides a power supply voltage varying from 7-8 volts and 17-18 volts, depending on the application.